Hertzian Crack Research Articles

Damage evolution in spark plasma sintered boron carbide under scratch and indentation loading conditions

Spark plasma sintered boron carbide (B4C) has been tested for damage evolution characteristics. Three distinct failure regions were identified during the scratch process of B4C, based on the observed transitions in the induced tangential force pattern and fracturing. B4C exhibited significant elastic recovery effect (50%) under both scratch (1–50 N) and indentations (5–100 mN) loading conditions. The sequence of transition from the nucleation of cracking and its propagation resulting to amorphization were identified. Scratch grooves were characterized with partial bow type Hertzian cracking, radial cracking and an onset of chipping (up to 75 N) and extensive spallation and fragmentation (76–100 N). The spallation regions were characterized with numerous randomly oriented slip lines suggesting failure dominated by the shear deformation. Raman spectra from spallation regions reveal peaks corresponding to amorphous B4C, inferring the occurrence of amorphization concentrated at the localized shear bands. At higher loads (>75 N), grain boundary fractures were observed in addition to simple cleavage fracture, which contributes for deteriorating material performance, by extensive cracking. The hardness and elastic modulus of B4C measured by nanoindentation were 44 GPa ± 6.6 GPa and 560 GPa ± 47 GPa, respectively.

Nanomechanical assessment of tribological behavior of TiN/TiCN multi-layer hard coatings deposited by physical vapor deposition

This research shows different hard Ti-based coatings deposited over steel substrates using the physical vapor deposition (PVD) method. The graded coating of the three produced Ti, TiN and TiCN were assessed using phase composition analysis and spectroscopy study. The microstructure characterization was studied by X-ray Photoelectron spectroscopy (XPS), Scanning Electron Microscopy (SEM), and transmission electron microscopy (TEM). The results show that integrating Ti interlayer with subsequent TiN/TICN layers can be considered a suitable way to yield hard Ti-based coating with enhanced mechanical and tribological behavior due to nitride and carbonitride phases formed from the breakdown of carbon and nitrogen atoms. The elastic modulus and hardness of all layers were identified by nanoindentation and utilized to achieve yield strength. Later, the curvature of the unloading step and elastic properties are used for finding flow behavior parameters in terms of work hardening factors (n and K). The attained findings extracted from the load–displacement curve of both nanoindentation and nano scratch were analyzed to examine the microscopic performance of all three Ti, TiN, and TiCN coatings. Scratch mechanisms analysis in all coatings shows that the main failures in samples are caused by tensile stress stemming from Hertzian cracks. However, as the force increases, the coating surface undergoes an incremental deformation, and failure mode varies to interfacial spallation.

Hertzian crack propagation in graded glass/ceramic composites

AbstractIn literature, the concept of material gradation is shown to inhibit surface crack initiation in glass/ceramic composites subjected to Hertzian indentation. However, surface cracks could yet initiate due to relatively higher loadings or in the presence of surface flaws/defects. Hence, characterization of graded composites concerning the resistance against Hertzian crack initiation and propagation manifests itself as a prominent matter. In this study, axisymmetric Hertzian cracks evolving in graded glass/ceramic composites propelled by a rigid cylindrical punch are investigated employing a novel recursive method, called the stacked‐node propagation procedure. Crack trajectories and their propagation susceptibilities are predicted via the minimum strain energy density (MSED) criterion regarding the crack growth resistance (R‐curve) of ceramics. The stress trajectory approach is also considered for a homogeneous glass to reveal the reliance and effectiveness of the MSED criterion in the present crack problems. The Mori–Tanaka relations are adopted to model the elastic modulus and Poisson's ratio variations through the composites, which are implemented on the simulations via the homogeneous finite element approach. Hertzian crack problem of a practically producible graded composite comprised of oxynitride glass and a fine‐grained silicon nitride ceramics (Si3N4) is treated as a case study. The degree of material gradation is assessed for the mitigation of surface crack initiation and propagation risks.

Evaluation of nanomechanical and nanoscratch response of Inconel 718 reinforced with graphene nanoplates produced by combined ARB-GTAW processes

An attempt was made to produce graphene reinforced Inconel 718 composite by new combining accumulative roll bonding (ARB) and gas tungsten arc welding (GTAW). The phase composition analysis and wear behavior of the composite were assessed. The microstructure characterization was studied by Raman spectroscopy, scanning and transmission electron microscopy. The results display that integrating ARB with GTAW can be consider as a suitable way to yield superalloy-based composite with enhanced tribological behavior due to carbide and carbonitride formation formed from decomposition of graphene nanoplates (GNPs). The elastic modulus and hardness were initial recorded by nanoindentation and then used to attain the yield stress, and subsequently for characterizing flow behavior parameters like n and K. The obtained results exposed that the load–displacement curve acquired from nanoindentation and Nanoscratching has an excellent potential to investigate microscopic behavior of particle based composites. Evaluation of scratch mechanisms in composite specimens presented that the key failure in specimens are due to tensile state Hertzian cracks. Though, as the applied load rises, the specimen undertakes increasing incremental plastic deformation and failures change to interfacial spallation.

An experimental study of the contact mechanical behaviour of marble spheres under cyclic loading

A series of cyclic compression tests of marble spheres and scanning electron microscopy (SEM) imaging measurements were performed in this work. The cyclic compression tests of marble spheres reveal three different fracture types: Hertzian cracks, oblique fractures and multi-meridian fractures. The displacement of marble spheres with Hertzian cracks or oblique fractures in cyclic compression tests can be significantly larger than the displacement (0.31 mm) corresponding to sample breakage in monotonic compression tests. The SEM observations reveal that mineral fragmentation, plastic shear sliding and crack closure occur in the local contact area, resulting in obvious residual displacement. Hertzian cracks were produced by the combination of densification and shear strength degradation under cyclic loading. Typical two-stage behaviour of cumulative residual displacement and dynamic secant stiffness evolution with the number of cycles was clearly observed. The occurrence of Hertzian cracks or oblique fractures produced significant cumulative residual displacement. The present laboratory results also show that the increase in amplitude can result in a significantly larger cumulative residual displacement and that the number of cycles of marble sphere failure in meridian fracture significantly increases with an increase in the frequency of cyclic loading. The upper contact force ratio and amplitude ratio significantly influence the dynamic secant stiffness evolution. The dynamic secant stiffness of tests under amplitude ratios of 0.7 and 0.8 shows a clear decreasing trend after the initial decelerating stage, which may be related to the loosening and decohesion of mineral particles caused by local significant fragmentation.

Acoustic signature identification of damage and wear mechanisms in a steel/glass sliding contact

The tribological behavior of a steel/glass ball-on-flat contact was studied by synchronizing the friction measurements with an acoustic emission device and a vision system. The results highlight two distinct friction regimes identified with low and high friction values. Their transition is characterized by a modification of acoustic emission signals. In addition, two main damage and wear mechanisms are identified: the creation and propagation of Hertzian cracks visible on the glass surface and the constitution of an interfacial layer of debris. The different accommodation mechanisms, activated successively or simultaneously, are identified for acoustic emission frequencies between 300 and 700 kHz. Eventually, this approach allows a real-time wear mechanisms identification and gives better insights about acoustic emission signals in relation to tribological systems.

Phase Field Modeling of Hertzian Cone Cracks Under Spherical Indentation

A phase field model of brittle fracture has been developed to simulate the Hertzian crack induced by penetration of a rigid sphere to an isotropic linear-elastic half-space. The fracture formation is regarded as a diffusive field variable, which is zero for the intact material and unity if there is a crack. Crack growth is assumed to be driven by a strain invariant. The numerical implementation is performed with the finite element method and an implicit time integration scheme. The mechanical equilibrium and the phase field equations are solved in a staggered manner, sequentially updating the displacement field and the phase field variable. Numerical examples demonstrate the capability of the model to reproduce the nucleation and growth of the Hertzian cone crack.

Tribolumen: A Tribometer for A Correlation Between AE Signals and Observation of Tribological Process in Real-Time—Application to A Dry Steel/Glass Reciprocating Sliding Contact

This paper deals with the development of an original apparatus called TRIBOLUMEN designed specifically for friction experiments on transparent materials. The friction measurement is synchronized with an acoustic emission (AE) sensor and the device is also equipped with a high-speed camera offering a direct view at the interface to gain a deeper understanding of tribological mechanisms. The TRIBOLUMEN device is in ball-on-flat contact configuration with a range of strokes from 5 to 500 µm and an oscillation frequency from 5 to 600 Hz. The experiments showed that this device has an adequate rigidity and can detect subtle friction modifications of the oscillating contacts. The observation of a steel-on-glass contact in real-time highlighted the initiation of Hertzian cracks followed by the formation of debris in the contact. Using the synchronous measurement, these mechanisms were clearly associated with different stages in the friction measurement and in the AE signals, which permitted to identify the AE signature of Hertzian cracks.

Numerical investigation of impact breakage mechanisms of two spherical particles

Recently developed peridynamic (PD) simulations, allowing spontaneous crack nucleation and propagation, have been conducted to investigate the dynamic breakage behavior of impact spherical particles as a function of impact speeds. With the increase of impact speed, the Hertzian cracks, primary meridian cracks, secondary meridian cracks, circumferential cracks, and crack bifurcations are observed. The crack propagation speeds can reach 62% Rayleigh wave speed at impact speed of 20 m/s. Then with the further increase of impact speed, crack bifurcation is observed due to dynamic path instability, leading to the catastrophic failure of impact spheres. The evolution of damage ratio, defined as the ratio of broken bonds to total initial bonds in PD model, agrees with previous results using discrete element method (DEM), validating the current model. Furthermore, the comparison between computed and theoretic contact force is performed along with experimental measurements. The research provides additional insights into breakage mechanisms of particles.

Velocity-dependent wear behavior of phosphate laser glass

Abstract Using a reciprocating sliding tribometer, the velocity-dependent wear behaviors of phosphate laser (PL) glass were investigated in dry and humid air. The experimental results show that the velocity dependence of wear in PL glass is very sensitive to the presence of water. In dry air, the velocity-dependent wear of PL glass shows fracture-dominated damage behavior. With increasing velocity, the Hertzian cracks increase first and then tend to saturation. Simultaneously, the material-removal volume also increases first and then keeps almost unchanged. However in humid air, the wear mechanism transforms into tribochemistry-controlled wear process, and almost no crack forms on glass surface for various velocities. With increasing velocity, the stress-enhanced hydrolysis becomes weaker and material-removal volume of PL glass decreases sharply. These results may help understand the surface damage and material removal of phosphate laser glass during machining and serving in various conditions.

Fabrication and investigation of the scratch and indentation behaviour of new generation Ni-P-nano-NiTi composite coating for oil and gas pipelines

Oil and gas pipelines operate under severe mechanical and chemical conditions, which could lead to catastrophic failure. Electroless nickel-phosphorous (Ni-P) coatings are an excellent candidate for protection against wear and corrosion for oil and gas pipelines. Ni-P coatings exhibit superior corrosion resistance over many commercially available coatings, as well as high hardness. However, electroless Ni-P coatings suffer from low toughness, which has limited its use in applications where high scratch and dent resistance is required. To enhance the toughness of the Ni-P coatings, nano-sized particles of superelastic NiTi alloy were incorporated within the coating. In the present work novel Ni-P-nano-NiTi composite coating has been successfully plated on API X100 pipe steel. Scratch tests under constant and increasing loads were performed to determine the effect of NiTi nano-particles on wear damages. Indentation tests were carried out to assess the dent resistance and crack modes of the coating. The addition of superelastic nano-particles into the Ni-P matrix led to toughening through transformation toughening, crack bridging, deflection, and microcracking. The composite coatings exhibited lower hardness in comparison to monolithic Ni-P coatings, but still provided considerable improvement upon the steel substrate. Optical and scanning electron examinations showed evidence of delamination, radial, and Hertzian cracking, as well as a reduction in crack length with the addition of superelastic NiTi.

Wear resistance of ZrB2 based ceramic composites

The wear resistance and tribological characteristics of spark plasma sintered ZrB2 + B4C, ZrB2 + SiC and ZrB2 + ZrC composites were investigated under dry sliding conditions at applied loads of 5 N and 50 N in air. The microstructure, deformation and damage characteristics were studied using scanning electron microscopy, confocal electron microscopy, a focused ion beam and atomic force microscopy. The friction coefficient values were very similar for all composites with values ranging from 0.63 to 0.72 and with the lowest value recorded for the ZrB2 + SiC composite at a 5 N applied load. The ZrB2 + ZrC composite was the most wear resistant, with wear rates at a 5 N load of 6.15 × 10−6 mm3/(N·m) and at a 50 N of 7.3 × 10−6 mm3/(N·m). None, or a very limited number of grain pull-outs and/or lateral fractures of grains were found during the wear tests. At the 5 N load, abrasive grooves connected with deformations and Hertzian crack formations were the main wear mechanisms in all systems, with limited crack formations in the ZrB2 + ZrC composite. Tribofilm formations connected with debris origin, oxidation and tribochemical reactions were dominant in all composites, with similar chemical compositions but different sizes and thicknesses at the 50 N load.

Investigation of nanomechanical and adhesion behavior for AlN coating and AlN/Fe2-3N composite coatings created by Active Screen Plasma Nitriding on Al 1050

This study investigated the effect of nitriding time and temperature on the mechanical properties of the composite AlN/Fe2-3N coating deposited on the pure aluminium substrate using the novel Active Screen Plasma Nitriding (ASPN) method. ASPN treatment was performed for 2, 5, 10, 15 h at temperatures of 450,500, and 550 °C and Conventional Plasma Nitriding (CPN) treatment was carried out for 5 h at 500 °C. All treatments were performed at the 10 KHz frequency with 80% duty cycle on the Al1050 substrate under 20%H2+80%N2 atmosphere. Phase and microstructure studies were performed using, respectively, the grazing incidence x-ray diffraction (GIXRD) system and the field emission - scanning electron microscopy (FE-SEM) system equipped with energy-dispersive spectroscopy (EDS) analyzer. The mechanical strength of the coating was evaluated by the roughness, nano-hardness, nano-scratch, and adhesion strength tests performed on coated and uncoated specimens. According to the results, the uncoated Al1050 specimen, the CPN-treated specimen, and the ASPN-treated specimen had a mean nano-hardness of 0.7 ± 0.1 GPa, 10.9 ± 0.6 GPa, and 9.6 ± 0.5 GPa, respectively. It was also found that the single-phase AlN coating has a better adhesion strength and scratch strength (LC3 = 12 N) than the composite AlN/Fe2-3N coating (LC3 = 10 N). Examination of scratch mechanisms in hard AlN and AlN + Fe2-3N coatings deposited on a soft aluminium substrate showed that the failures in these coatings are due to tensile-type Hertzian cracks. However, as the load increases, the substrate undergoes increasing deformation and failures shift to chipping and interfacial spallation.

Two-Dimensional Numerical Simulation of Rock Fragmentation by TBM Cutting Tools in Mixed-Face Ground

In the study described in this paper, a general particle dynamics (GPD) code was developed to simulate rock fragmentation by tunnel-boring machine (TBM) cutters in mixed-face ground. In the GPD code, a novel algorithm of the contacts at the boundary between two different media was proposed. The mechanism of rock fragmentation by two disc cutters and the effects of cutter spacing on rock fragmentation in mixed conditions were investigated. With increasing cutter spacing, the propagation pattern of Hertzian cracks varied and an optimal cutter spacing was found. The mechanism of rock fragmentation by a disc cutter and a drag bit and the effects of the acting position of different TBM cutters on rock fragmentation in mixed conditions were studied. The optimal acting position of the disc cutter and the drag bit was found to benefit the effectiveness of TBM rock fragmentation.

Effect of tooth substrate and porcelain thickness on porcelain veneer failure loads in vitro

Statement of problemBonded porcelain veneers are widely used esthetic restorations. High success and survival rates have been reported, but failures do occur. Fractures are the commonest failure mode. Minimally invasive or thin veneers have gained popularity. Increased enamel and porcelain thickness improve the strength of veneers bonded to enamel, but less is known about dentin or mixed substrates. PurposeThe purpose of this in vitro study was to measure the influences of tooth substrate type (all-enamel, all-dentin, or half-dentin-half-enamel) and veneer thickness on the loads needed to cause initial and catastrophic porcelain veneer failure. Material and methodsModel discoid porcelain veneer specimens of varying thicknesses were bonded to the flattened facial surfaces of incisors with different enamel and dentin tooth substrates, artificially aged, and loaded to failure with a small sphere. Initial and catastrophic fracture events were identified and analyzed statistically and fractographically. ResultsFracture events included initial Hertzian cracks, intermediate radial cracks, and catastrophic gross failure. All specimens retained some porcelain after catastrophic failure. Cement failure occurred at the cement–porcelain interface not at the cement–tooth interface. Porcelain veneers bonded to enamel were substantially stronger and more damage-tolerant than those bonded to dentin or mixed substrates. Increased porcelain thickness substantially raised the loads to catastrophic failure on enamel substrates but only moderately raised the loads to catastrophic failure on dentin or mixed substrates. The veneers bonded to half-dentin-half-enamel behaved remarkably like those bonded wholly to dentin. ConclusionsPorcelain veneers bonded to enamel were substantially stronger and more damage-tolerant than those bonded to dentin or half-enamel-half dentin.

Damage progression in silicon nitride undergoing non-conforming hybrid cyclic contact

Bearings experience one of the most severe mechanical loading of all machine elements. The contact stresses engendered are highly localised and bound to a very small volume of the material. The aim of this study was to investigate how localised stresses influence the damage mechanism in hybrid contact. Cyclic contact loading of a gas pressure sintered silicon nitride (GPSN) was investigated. Silicon nitride disks and tungsten carbide (WC) indenters were tested under different media, initially at “application relevant” low contact pressures (4–6GPa) and further on, to accelerate damage, at high contact pressures (10–15GPa). The low load experiments showed various forms of surface damage with no significant difference between dry and lubricated contact. Whereas, the high load experiments showed different damage behaviour under unlubricated and lubricated conditions. Unlubricated contact resulted in the formation of transfer layers and Hertzian cracks on the silicon nitride surface whereas, damage under lubricated contact was mainly dominated by grain removal and delayed crack formation. Finite element simulations were carried out to study the stress state under different loading conditions. The FEM results indicated that the combination of surface tensile and shear stresses predominantly influence the fatigue damage observed in the experiments rather than fluctuating tensile stresses only.

Elastic stress field model and micro-crack evolution for isotropic brittle materials during single grit scratching

Abstract An analytical model for the elastic stress field in isotropic hard and brittle materials during scratching is presented. The model considers the entire elastic stress field and the effect of material densification that was ignored in past studies, and is developed under a cylindrical coordinate system to make the modeling process simpler. Based on the model's predictions, the location and sequence of crack nucleation are estimated and the associated mechanisms are discussed. A single grit scratching experiment with an increasing scratch depth up to 2 µm is conducted for two types of optical glasses representing isotropic brittle materials: fused silica and BK7 glasses. It is found that the model's predictions correlate well with experimental data. Median cracks are found to be formed first during scratching, and the corresponding depth of the scratch sets the basis for determining the critical depth for brittle to ductile machining. Lateral cracks are initiated in the plastic yielding region and deflect to the work surface to cause material removal, while Hertzian cracks interact with lateral cracks to help remove lateral-cracked material. Furthermore, it is found that, owing to its open network molecular structure, fused silica has a much worse ductile machinability than the BK7 glass.

Experimental and numerical investigation of crack initiation and propagation in silicon nitride ceramic under rolling and cyclic contact

The focus of the work was to investigate crack initiation and propagation mechanisms in silicon nitride undergoing non-conforming hybrid contact under various tribological conditions. In order to understand the prevailing modes of damage in silicon nitride, two distinct model experiments were proposed, namely, rolling contact and cyclic contact experiments. The rolling contact experiment was designed in order to mimic the contact conditions appearing in hybrid bearings at contact pressures ranging from 3 to 6 GPa. On the other hand, cyclic contact experiments with stresses ranging from 4 to 15 GPa under different media were carried out to study damage under localised stresses. In addition, the experimentally observed cracks were implemented in a finite element model to study the stress redistribution and correlate the generated stresses with the corresponding mechanisms. Crack propagation under rolling contact was attributed to two different mechanisms, namely, fatigue induced fracture and lubricant driven crack propagation. The numerical simulations shed light on the tensile stress driven surface and subsurface crack propagation mechanisms. On the other hand, the cyclic contact experiments showed delayed crack formation for lubricated cyclic contact. Ceramographic cross-sectional analysis showed crack patterns similar to Hertzian crack propagation under cyclic contact load.

Effect of water on wear of phosphate laser glass and BK7 glass

Using a reciprocating ball-on-flat tribometer, the friction and wear behaviors of phosphate laser (PL) glass and BK7 borosilicate glass were investigated by rubbing against Al2O3 ball in dry air and liquid water to reveal the effect of water on the wear of the two glasses. In dry air, the wear of PL glass and BK7 glass shows a typical characteristic of mechanical wear. The damage of PL glass dominates by the formation and extension of Hertzian crack, and the damage of BK7 glass dominates by exfoliation-in-chips and three-body abrasive wear. Water affects both the subsurface fracture and material removal for the two glasses. On the one hand, the subsurface fracture is suppressed greatly for both the PL glass and BK7 glass when they are rubbed in water. The lower shear stress caused by water film at friction interface is considered as the main reason of weakened subsurface cracking for the two glasses. On the other hand, compared with dry air, water promotes the material removal of PL glass but reduces the material removal of BK7 glass. It suggests that the material removal of glass in water depends on the combined effect of the chemical stability of glass and the contribution of water lubrication.

Mechanical damage of hard chromium coatings on 416 stainless steel

Abstract Hard chromium coatings are widely used for high performance components due to their superior wear and corrosion resistance. In this study, 416 stainless steel bars were chromium electroplated commercially with different thickness. Mechanical properties and residual stress were measured using a nano-indentation system and X-ray diffraction, respectively. Image analysis technique was used to measure inherent crack density. Residual stress in the coating increases with increasing thickness leading to an increase in inherent crack density. An indentation study was carried out to investigate the mechanical damage of hard chromium coating as a function of thickness. Different failure modes were observed when hard chromium coatings were indented with a spherical indenter including Hertzian cone cracks, radial cracks, bend cracks, lateral cracks and coating delamination. Three thickness regions of distinctive cracking modes were identified. Thin coating region exhibited through-thickness Hertzian cracks. Hertzian and coating-bend rings, cone, surface radial cracks and coating bottom surface radial cracks were observed in intermediate coating region. This region exhibited highest cracking damage density. In thick coatings Hertzian cracks were observed. It was found that two factors affect the cracking damage: residual stress that increases with increasing thickness and bending stresses that decrease with increasing thickness. The resultant effect was maximized at intermediate coating thickness.